The present invention generally relates to catalytic layers and, more particularly, to a method of preparing a catalytic layer on a metallic surface, such as the inside of a recuperator.
Using a metallic substrate to support a catalytic coating continues to become more important. Due to its high thermal conductivity and good ductility, metals have been found to be a preferred choice of catalyst substrates in applications where robust heat transfer, complicated flow path, and lightweight are critically important. One such application is a metallic catalytic heat exchanger (such as a recuperator) where a part of or the whole heat-transfer area can be functionalized with catalytic properties by applying a catalyst layer over the metal substrate. To prepare a catalyst coating over a variety of metal substrates with a simple, generalized method, however, represents major technical challenges.
Generally, the catalyst layer typically consists of a small fraction of an active metal ingredient supported by a refractory metal oxide with high surface area. A traditional method of preparing the metal oxide layer as the catalyst support has been to make a refractory metal oxide slurry with water and acid, and then washcoating the resulting mixture directly over a ceramic substrate such as a cordierite monolith. An example of this approach is the acid stabilized alumina sol method. This method generally cannot be used directly for a metal surface because of weak coating binding strength that results in spalling and peeling of the coated catalyst layer.
Currently, there are a few methods of coating a metal substrate with a high surface area refractory metal oxide before applying the catalytic metal ingredients. However, these methods usually have significant limitations and are applicable to only one or a few metal-type surfaces. One such method is to coat an alumina slurry to the surface of a stainless steel containing aluminum (Al) as a key component (e.g. Fecralloy and Kanthal, etc.). The surface, however, needs to be pretreated with high temperature air so that a layer of alumina or alumina whiskers can be formed before the catalyst coating.
Another past method includes forming an underlayer with a refractory metal oxide slurry before coating an overlayer of catalyst. The underlayer consists of mixture of alumina with silica sol in order to improve the binding strength of the catalyst overlayer coating.
A further past method provides an underlayer by coating a mixture comprising a dispersion of aluminum metal powder in an aqueous solution of a chromium salt and aluminum phosphate, followed by curing at elevated temperatures.
Some disadvantages, however, to the above methods include the fact that using an aluminum-containing alloy substrate significantly limits the choice of metal substrate for application of the catalyst. Also, the types of stainless steel substrates having such alloys are generally expensive. Additionally, forming alumina whiskers or an oxide layer adds cost and time associated with the process. The use of chromium salt, a known carcinogen, generates an environmental hazard. Phosphate is known to deactivate catalyst function by migrating from the underlayer and reacting with catalytic metal under elevated temperatures.
For certain applications, the substrate metal may be something other than stainless steel, such as titanium. Yet, needing to use a silica sol mixture as an underlayer not only increases the number of catalyzing process steps but can also result in undesirable catalytic effects known as silica poisoning, which can deactivate the catalyst under elevated reaction temperature. Furthermore, the binding strength of the undercoat layer containing silica can be significantly weakened under high temperature and high humidity due to hydrolytic scission of silicon-oxygen bond, resulting in spalling and peeling of the coating.
It is, therefore, highly desirable to have a catalytic coating method for metal substrates which eliminates the above limitations.
Accordingly, in one aspect of the present invention, a method of catalytically coating a metal surface comprises coating the metal surface with a slurry containing a binder; forming an oxide coating on the metal surface; and applying a solution of catalytic material onto the oxide coating.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims.